Forensic toxicology is the science of detecting drugs, poisons, and other chemicals in biological samples to support legal investigations. It plays a central role in death investigations, impaired driving cases, workplace drug testing, and criminal prosecutions where substances may have influenced someone’s behavior or caused harm. The field sits at the intersection of chemistry, pharmacology, and law, and its findings can determine whether a death is ruled an overdose, whether a driver was impaired, or whether someone was drugged against their will.
The Three Branches of Forensic Toxicology
Forensic toxicology breaks into three main specialties, each focused on a different type of question.
Postmortem toxicology examines biological samples collected during an autopsy to determine whether a substance caused or contributed to someone’s death. This is the branch most people picture when they think of forensic toxicology: a medical examiner sends tissue and fluid samples to a lab, and a toxicologist identifies what substances were present and at what concentrations.
Human performance toxicology focuses on how substances affect a living person’s ability to function. The most common application is impaired driving cases, where a toxicologist determines whether alcohol, cannabis, prescription drugs, or other substances were present at levels that could impair a driver’s coordination, reaction time, or judgment.
Forensic drug testing covers standardized testing programs like workplace drug screens, probation monitoring, and military testing. These follow strict protocols to ensure results hold up legally.
Biological Samples and Why They Matter
The type of sample a toxicologist analyzes depends on the situation and what question needs answering. Blood and urine are the traditional workhorses, but they’re far from the only options.
Blood is the preferred specimen for impaired driving investigations because drug levels in blood correlate most closely with what’s actively affecting the brain at or near the time of collection. Urine, by contrast, only shows that a substance was used at some point in the recent past. A positive urine test doesn’t mean someone was impaired when the sample was taken, which is why expert consensus panels strongly encourage blood or oral fluid over urine for impairment cases.
Oral fluid (saliva) has become the most widely accepted alternative for assessing recent drug exposure. Collection is simple, noninvasive, and painless, and drug levels in saliva tend to reflect the concentration of free, active drug circulating in the bloodstream.
Hair provides a much longer detection window, capturing a record of drug exposure stretching back months or even years. In postmortem cases where a body has badly decomposed, hair may be the only usable specimen. Vitreous humor, the gel-like fluid inside the eye, is particularly valuable when a body is burned, embalmed, or heavily decomposed, because it’s naturally isolated from many of the chemical changes that affect blood after death.
How Testing Works: Screening and Confirmation
No single test can detect every possible substance, so forensic toxicology labs use a layered approach. The process typically starts with broad screening and narrows down to precise identification.
Initial screening uses rapid methods, often immunoassay-based tests, that flag categories of drugs (opioids, amphetamines, cannabinoids, benzodiazepines, and so on). These tests are fast and relatively inexpensive, but they can produce false positives. A screening test might react to a cold medication the same way it reacts to an illicit stimulant.
That’s why forensic cases always require confirmatory testing. The gold standard is gas chromatography paired with mass spectrometry, a technique that separates chemical compounds and then identifies them by their unique molecular fingerprint. Liquid chromatography with tandem mass spectrometry is increasingly common as well, especially for substances that don’t vaporize easily. These instruments can detect drugs at concentrations measured in billionths of a gram per milliliter of blood.
For toxic metals like arsenic or lead, a different type of mass spectrometry designed for elemental analysis can detect trace amounts down to the low parts-per-trillion range. Labs may also use high-resolution instruments that combine multiple analytical techniques to identify completely unknown compounds, which is critical when dealing with new synthetic drugs that don’t show up on standard panels.
Interpreting Results: More Than Just Detection
Finding a drug in someone’s system is only the first step. The harder question, and the one that matters in court, is what that drug was doing to the person. This is where interpretation becomes both science and art.
For well-studied substances, toxicologists compare detected concentrations against established ranges. Morphine in blood, for example, is considered therapeutic between 0.02 and 0.1 mg/L. Above 0.2 mg/L, it enters toxic territory and may indicate misuse or overdose. Diazepam (a common benzodiazepine) is generally therapeutic below 0.5 mg/L but can become lethal above 5 mg/L, particularly when combined with alcohol or other depressants.
These thresholds are guidelines, not hard rules. A person with long-term opioid tolerance might function normally at concentrations that would kill someone else. Polydrug use complicates things further: fentanyl is often cited as lethal above 3 nanograms per milliliter of blood, but deaths have occurred at lower levels when other depressants were also present.
Novel psychoactive substances and constantly evolving synthetic drugs make interpretation even more challenging, because there simply isn’t enough published data on their lethal or impairing concentrations yet.
The Problem of Postmortem Redistribution
One of the trickiest challenges in death investigations is that drug concentrations in the body change after death. This phenomenon, called postmortem redistribution, means the levels measured at autopsy may not reflect what was in the person’s blood while they were alive.
After death, drugs stored in organs like the heart, lungs, and liver can leak into surrounding blood and tissues through passive diffusion. Blood clotting, the settling of blood due to gravity, and bacterial decomposition all further alter drug concentrations. Drugs that are highly fat-soluble or that accumulate heavily in tissue tend to redistribute the most.
This is why toxicologists prefer to collect blood from the femoral vein in the leg rather than from the chest cavity, where proximity to drug-rich organs can inflate readings. It’s also why interpreting postmortem results demands deep expertise in pharmacology, not just chemistry.
Forensic Toxicology in Impaired Driving Cases
Alcohol remains the most commonly tested substance in impaired driving investigations, with most labs using a reporting threshold of 0.010 g/dL, well below the 0.08 g/dL legal limit in most U.S. states. But drug-impaired driving has grown into an equally significant concern. In a survey of 64 forensic laboratories, THC and its metabolites were the most frequently detected drug class, reported by nearly every lab.
The substances that concern toxicologists in driving cases are those with pharmacological effects that directly impair driving ability: sedation, drowsiness, slowed reaction time, overstimulation, hallucinations, and distorted judgment. These include prescription sleep aids, benzodiazepines, opioids, stimulants, and cannabis. Drug withdrawal can also impair driving, even when the drug itself is no longer detectable.
Blood is the specimen of choice because it reflects what’s circulating in the body near the time of driving. Oral fluid is an acceptable alternative. Urine is considered unreliable for impairment cases, though some states still allow or require it.
The Role of Expert Testimony
Forensic toxicologists frequently testify in court as expert witnesses. Before they can offer opinions, they must be formally qualified through a process called voir dire, where attorneys question them about their education, training, experience, publication record, and prior testimony. In forensic science specifically, the questioning often extends to proficiency testing results, laboratory accreditation, the types of instruments they work with, and their supervisory experience.
Their testimony typically covers what substances were found, at what concentrations, and what effects those substances would be expected to produce. In death investigations, they may opine on whether a substance caused the death, contributed to it, or was incidental. In impairment cases, they explain how detected drug levels relate to the behavioral effects observed by law enforcement.
Professional Standards and Certification
The American Board of Forensic Toxicology (ABFT) has been certifying forensic toxicologists since 1975. Certification requires a combination of education, specialized training, professional experience, and passing a formal written examination. Applicants must be actively working in forensic toxicology.
The ABFT also accredits laboratories that perform postmortem and human performance toxicology, setting standards for quality and competence. It partners with the ANSI National Accreditation Board (ANAB) to advance laboratory accreditation and has been accredited by the Forensic Specialties Accreditation Board since 2006. These layers of oversight exist because forensic toxicology results carry legal consequences, and errors can mean wrongful convictions or missed justice.